1. Field of the Invention
This invention relates to a device for compensation for changes in the output of a photodetector caused by intensity nonuniformity of a scanning light beam, scanning speed nonuniformity, light guiding nonuniformity of a light guiding optical system connected to the photodetector, sensitivity nonuniformity of the photodetector, or the like in an image read-out apparatus for scanning a recording medium carrying an image recorded thereon with the light beam and photoelectrically detecting light carrying the image information by the photodetector. This invention also relates to a device for compensation for the aforesaid changes in an image read-out and reproducing system comprising the aforesaid image read-out apparatus and an image reproducing apparatus for reproducing the image based on a read-out image signal generated by the image read-out apparatus.
2. Description of the Prior Art
Image read-out apparatuses for reading out an image by scanning a recording medium carrying the image recorded thereon with a light beam and detecting light reflected by the recording medium, light transmitting through the recording medium, or light emitted by the recording medium have heretofore been used, for example, as image input sections for computers and image read-out sections of facsimiles. In the image read-out apparatus of this type, the light beam emitted by a laser beam source or the like is deflected by a light deflector such as a galvanometer mirror to scan the recording medium, and light obtained by the recording medium and carrying the image information is detected by the photodetector. In general, in order to detect the light efficiently, a light guide member fabricated of a light guiding material is used, one end thereof is constituted as a light input end face, and the other end (light output end face) thereof is closely contacted with a photodetector such as a photomultiplier for detecting the light carrying the image information. A light guiding mirror may be positioned in the vicinity of the light input end face of the light guide member for reflecting light, which does not advance towards the light input end face, towards the light input end face, thereby improving the light guiding efficiency.
However, in the aforesaid image read-out apparatus, the output of the photodetector may change due to intensity nonuniformity caused by nonuniformity of the reflectivity of the reflecting surface of the light deflector, scanning speed nonuniformity of the light beam caused by fluctuations in the deflecting speed of the light deflector, or light guiding nonuniformity of the light guide member, i.e. nonuniformity of the light guiding efficiency, for example, at the end portion of the light guide member. In the case where the light guiding mirror is provided, reflecting and light guiding nonuniformity may also be caused by stains on the mirror or the like. Also, the photomultiplier or other photodetector may involve sensitivity nonuniformity wherein the sensitivity is different at different positions on the light receiving face. When regional deterioration of the light detecting efficiency (i.e. shading) is caused by the aforesaid nonuniformities, it becomes impossible to accurately detect the light obtained from the recording medium.